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General Chemistry Principles, Patterns, and Applications, 2011

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atoms held together by covalent bonds; they tend to be very hard <strong>and</strong> have high melting points. Metallic<br />

solids have unusual properties: in addition to having high thermal <strong>and</strong> electrical conductivity <strong>and</strong> being<br />

malleable <strong>and</strong> ductile, they exhibit luster, a shiny surface that reflects light. An alloy is a mixture of<br />

metals that has bulk metallic properties different from those of its constituent elements. Alloys can be<br />

formed by substituting one metal atom for another of similar size in the lattice (substitutional alloys),<br />

by inserting smaller atoms into holes in the metal lattice (interstitial alloys), or by a combination of<br />

both. Although the elemental composition of most alloys can vary over wide ranges, certain metals<br />

combine in only fixed proportions to form intermetallic compounds with unique properties.<br />

K E Y T A K E A W A Y<br />

<br />

Solids can be classified as ionic, molecular, covalent (network), or metallic, where the<br />

general order of increasing strength of interactions is molecular < ionic ≈<br />

metallic < covalent.<br />

C O N C E PTUAL P R OBLEMS<br />

1. Four vials labeled A–D contain sucrose, zinc, quartz, <strong>and</strong> sodium chloride, although not necessarily in that<br />

order. The following table summarizes the results of the series of analyses you have performed on the<br />

contents:<br />

A B C D<br />

Melting Point high high high low<br />

Thermal Conductivity poor poor good poor<br />

Electrical Conductivity in Solid State moderate poor high poor<br />

Electrical Conductivity in Liquid State high poor high poor<br />

Hardness hard hard soft soft<br />

Luster none none high none<br />

2. Match each vial with its contents.<br />

3. Do ionic solids generally have higher or lower melting points than molecular solids? Why? Do ionic solids<br />

generally have higher or lower melting points than covalent solids? Explain your reasoning.<br />

4. The strength of London dispersion forces in molecular solids tends to increase with molecular mass, causing a<br />

smooth increase in melting points. Some molecular solids, however, have significantly lower melting points<br />

than predicted by their molecular masses. Why?<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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